1. Field of the Invention
Extrusion system comprising a screw-changing device The invention relates to an extrusion system comprising at least one screw-type extruder whose at least one screw is drive-connected via a coupling to a gearing that is driven by a motor, said coupling being able to be coupled to and decoupled from the screw.
2. Description of the Related Art
Extrusion systems are used particularly in the plastics industry for production of plastic products and also for compounding of plastics, i.e. for manufacturing intermediate products made up of different substances. Extrusion systems for the latter purpose are also referred to as compounding systems and in many cases have extruders with two or more screws which mesh with one another. In these compounding systems, additives such as dyes or fiber materials, for example, are mixed into the plastics raw material and distributed as uniformly as possible therein. Given the wide variety of thermoplastics in particular, and the wide variety of additives, which have different material properties in their combination, optimum production is sought by .using screws which, in their geometric configuration, are adapted as much as possible to the particular properties of the material which is to be processed. There are also screws which are comparatively well suited for the processing of a relatively broad range of materials. However, this means that, in the processing of the individual materials, such a screw or pair of screws generally has to operate some way from the actual operating optimum. This may entail a corresponding decrease in product quality and/or a reduced production rate. In large-scale systems in particular, with high throughput rates, special screws are therefore often used which are optimally suited for the processing of the particular material, in other words guarantee a very high quality at a very high production rate.
For processing plastics batches of different materials in smaller quantities, the use of special screws for the particular material involves a disproportionately high operating outlay since changing the screws has hitherto generally required a relatively high cost, particularly in the form of the shutdown of the production plant. In many operations, it is therefore customary to process smaller charges of different substances on separate machines which generally have a lower output rate than the large-scale systems for main production quantities. These separate machines not only take up a corresponding space, but are also in many cases characterized by a relatively low degree of utilization. This ultimately means that the processing of smaller batches is relatively expensive compared to the main production quantities of the standard products.
Moreover DE 3516311 C2, which corresponds to U.S. Pat. No. 4,678,343, discloses a device with a large number of molding stations arranged on a turntable, and with a jet-molding unit which can be docked onto these molding stations and with which polyurethane shoe soles are injected onto shoe uppers. The material components for the liquid polyurethane are in this case mixed in a mixing head provided with a short mixing screw which is mounted at one end and rotates at high speed (15000 to 18000 rpm). In order to change the mixing screw, the mixing-screw bearing and drive mechanism can be drawn back by means of a hydraulic cylinder system which primarily executes the displacement movement required for docking the mixing head onto the molding stations, so that the mixing screw is withdrawn completely from the mixing head before it is decoupled from the drive mechanism and replaced with another one by means of a hydraulically operated screw-changing device. In the case of longer screws, this solution would require an extremely long construction space. In contrast to jet-molding units, such docking movements are not customary in extrusion systems of the aforementioned type and, instead, the downstream equipment remains unchanged in its spatial arrangement with respect to the screw extruder during production.
Moreover, DE 3643884 A1, which corresponds to U.S. Pat. No. 5,020,914, discloses a screw-type injection molding machine in which, by means of an injection cylinder, the whole injection unit on a base frame of the injection molding machine can be advanced to the injection-molding tool. To change the screw of the injection unit, the whole drive mechanism of the screw can be drawn a short distance back from the injection cylinder on the base frame, until the screw is decoupled from the drive mechanism. To be able to remove the screw from the screw cylinder of the injection unit, the screw cylinder is pivoted about a vertical axis of rotation so that the screw can be drawn past the drive mechanism in an oblique position. For pivoting of the screw cylinder, the latter is connected to a special pivot bearing arranged for this purpose on the side.
The object of the present invention is to develop an extrusion system of the generic type in such a way as to substantially reduce the outlay required for converting the extrusion system from processing of one material to processing of another material with different properties. The aim therefore is to permit rapid changing of material while at the same time maintaining a high product quality and high productivity.
For an extrusion system of the generic type, the gearing is mounted on a gearing carriage configured to draw the gearing back a short distance in the direction of the axis of the screw in order to decouple the coupling. The carriage can be pivoted or can be moved transversely to the axis of the screw so that the gearing with the coupling can be moved away from the screw so that the screw can be withdrawn completely from the extruder counter to the feed direction of the screw. A screw-changing device is provided by which the existing screw of the extruder can be received and can be replaced by another screw in the screw-changing device.
The term xe2x80x9cgearing carriagexe2x80x9d should not be understood here in the narrow sense that such a gearing carriage must in fact also be provided with wheels for its movement. It could also, for example, be a sled which slides on runners in corresponding guides. However, the gearing carriage is preferably provided with rollers or wheels for its movement.
In principle, the gearing carriage could also be designed as a swivel unit so that, after the loosening of the fastening elements (e.g. screws) with which the gearing is connected securely to the extruder, said gearing with the coupling is pivoted out to the side for example, so that at the same time the coupling to the screw is decoupled and the rear space for drawing the screw back counter to its feed direction is made available. Although pivoting to the side is preferred, the pivot movement could in principle also be upward or downward.
The gearing carriage is preferably designed such that the gearing on the gearing carriage can be drawn back in the direction of the longitudinal axis of the screw in order thereby to disconnect the coupling of the screw. The gearing carriage itself can expediently be moved transverse to the longitudinal axis of the screw. To permit disconnection of the coupling, the gearing carriage can be equipped for example with a small sled whose runners extend parallel to the feed direction of the screw. The two movements for disconnecting the coupling and for moving the gearing away, with the drive motor flange-mounted thereon, could in principle also be made possible by providing the gearing carriage with wheels, for example guidable wheels.
The screw-changing device for mechanized exchange of the screw can be designed as a stationary or movable screw magazine, with a maneuvering device for withdrawal and insertion of a screw. In the simplest case, the screw magazine is designed with a rotatable double arm, where one arm is intended to receive the screw to be replaced, and the other arm carries the new screw. The screw magazine is preferably configured as a revolver magazine or paternoster magazine and is thus suited to receive a large number of replacement screws. This ensures a high degree of flexibility in terms of the processing of a correspondingly large number of different materials.
The maneuvering device for removing and inserting a screw is preferably a linearly movable slide with a gripping device for connection to the particular screw.
In principle it is possible for the movements of the gearing carriage and of the maneuvering device to be executed by an operator. In this case, the configuration of the extrusion system according to the invention relieves the operator at least from the heavy work during dismantling of the old screw and fitting of the new screw. In addition, the necessary movements of the screws can take place in set tracks, so that the time needed for screw-changing is considerably reduced.
Still greater time savings can be made if the gearing carriage and if appropriate also the screw-changing device are each provided with a motor drive. This drive is preferably actuated by an electronic control means whose program memory is preferably set up for fully automatic screw-changing.
A particularly significant saving is achieved if an extrusion system equipped with twin-screw extruders is designed in accordance with the invention, in which case the screw-changing device is set up for screw-changing in pairs. Such extrusion systems are particularly advantageous as compounding systems for processing of thermoplastics.